What does the term 'minimum en route altitude (MEA)' refer to?

The term 'minimum en route altitude (MEA)' refers specifically to the altitude that guarantees navigational signal coverage along a specific route. It ensures that aircraft are able to receive reliable navigation signals from ground-based navigation aids while flying en route. This altitude takes into account terrain clearance as well as any obstacles that may exist in the vicinity of the flight path, thereby enhancing safety during flight.

While the other options discuss important altitudes, they do not accurately describe the MEA. The highest altitude for safe flight is a general statement that could refer to various levels, but the MEA has a specific purpose related to signal coverage. Environmental limits pertain to different considerations such as atmospheric conditions and regulations, which are not the central focus of MEA. The average cruising altitude refers to a common altitude for different phases of flight, rather than a defined minimum for route navigational safety. Thus, the MEA is uniquely defined by its role in ensuring effective navigational signal coverage, making it the correct identification of the term.